While other nations are pouring more money than ever into scientific drilling projects that reach deep into the earth's crust, researchers at the deepest scientific drillhole in the United States, located near the San Andreas fault, are packing up their instruments. They are preparing for next week, when funding cuts will force the end of drilling operations there for at least two years, if not permanently. Now at a depth of 3 1/2 kilometers, the hole at Cajon Pass, Calif., is still some 1 1/2 km shy of its original goal, and San Andreas experts are wondering if 3 1/2 km is deep enough to resolve a 20-year-old paradox about the fault.

"The whole purpose of this experiment was to do something definitive, which meant going deep enough," says Stanford University's Mark Zoback, who is chief scientist on the Cajon Pass project.

Although the hole has provided some answers about the earthquake-producing forces along the San Andreas, the researchers have yet to reach the important depths between 5 and 10 km, where great California quakes are born.

Probing these depths would require about $12 million a year over the next two years to finish the project, according to Zoback. But for fiscal year 1988, the project received $4.8 million instead of the $6 million the scientists had expected from the National Science Foundation's Continental Lithosphere Program. And next year, fiscal year 1989, that program will be giving priority to other projects that were not favored this year, and the drillhole will receive significantly less funding. Therefore, if drilling does begin again, it must wait for fiscal 1990.

Zoback and many other project scientists have been working at Cajon Pass to examine the forces that generate the earthquakes along this fault, which is at the boundary of the Pacific and North American plates -- two huge sections of the earth's crust that are slowly slipping past each other.

Earthquakes happen because at some spots the rocks on opposite sides of the fault jam. Friction between the rocks causes the fault to lock for years or centuries, until the stress becomes too great and the rocks suddenly slip, generating the seismic waves of an earthquake.

The paradox of the San Andreas revolves around the stress on the rocks of the fault (SN: 1/31/87, p.70). Scientists have traditionally believed that the fault is strong, meaning that stress along the fault is relatively high. Generations of laboratory experiments and theories are based on this supposition.

But evidence in the last 20 years has caused scientists to question the strong fault theory. A high-stress fault should generate heat. Yet experiments in hundreds of shallow boreholes have not detected the expected high temperatures. Although these tests suggest the fault is weak, scientists could not be sure of the results because the holes were too shallow.

The Cajon Pass project was meant to yield a definitive answer to this question, and, says Zoback, "everything we've found so far is highly indicative of low stress on the fault."

If the fault is weak, then scientists will have to recast their theories about how the San Andreas operates. They will need new laboratory experiments and perhaps future drillholes to determine why the frictional forces along the fault are so low. Pressurized groundwater or a layer of clay filling the fault may be lowering the friction and permitting the fault to move under low stresses.

While this revolution in thinking will not affect ideas about the destructiveness of earthquakes along the San Andreas, "it may be that we're using the wrong kinds of models for earthquake prediction," says Zoback.

But those who study the San Andreas are not sure whether the present hole is deep enough to serve as a basis for firm conclusions about the fault. Because the stress experiments require a stable area, the hole was placed 3 1/2 km east of the fault. Because of this distance from the actual fault, the hole ideally should be dug significantly deeper than 3 1/2 km, according to the researchers, in order to obtain accurate results:

"The impact of whether the fault is weak is so enormous," says Zoback. "Do we undertake this revision in thinking without being 100 percent sure of what we're talking about?"

Zoback and most other scientists on the project want to reach the target depth to be sure. "For the purposes of the heat flow experiments, it is necessary to get to 16,000 feet (almost 5 km)," says Lee Silver of Caltech in Pasadena, who is investigating temperatures around the fault.

It is unclear, however, what will happen when the lithosphere program again receives enough funds to support the nation's scientific drilling program. The Cajon Pass hole is the first project of the program, and it has received top priority in the last year and a half. When funding returns, the planning committee will have to decide whether to return to Cajon Pass or start other projects that have been put on hold.

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